Discharge tube



Jan. 15, 1935 D. v. EDWARDS AL 1,987,998

DI SCHARGE TUBE Filed July so, 1952 INVENTOR S DWI! M zdwm MM/(MPatented Jan. 15, 1935 UNITED STATES PATENT OFFlCE DISCHARGE TUBE DonaldV. Edwards, Montclair, and Earl K. Smith, ,East Orange, ,N. J.,assignors to Electrons, Inc.

of Dela'waraa corporation of Delaware Application July 30, 1932, SerialNo. 626,619 6 Claims. (Cl. ate- '15) This invention relates to gaseousdischarge tubes andis of peculiar'utility in connection with the type ofdischarge tube known as control rectifiers in which the starting ofthe'gaseous' discharge betweenthe cathode and anode is controlled by athird element.

In this type of rectifierit is important that the characteristics of thetube shall remain'uniform throughout its life and also among similartubes when such tubes are manufactured in quantity. It is of especialimportance that the starting of the tube shall, for a given potentialapplied to the grid, occur at the same point on the curve ofcathode-anode voltage in tubes which are in tended to replace othertubes of like construction, and that such point shall not be subject toshifting during the life of the tube, especially when the tube has stoodidle without load, but with filament lighted for some time.

One of the causes of such shifting of control and variation ofcharacteristics is the deposit of emissive materials such as barium,strontium, calcium, sodium, or their salts, upon the surfaces ofnormally non-emissive electrodes, such as grid or plate.

Since barium, strontium or calcium is usually a constituent of thecathode, and sodium salts are often present in the electrodes or gases,it is desirable that they be prevented from depositing on thenon-emissive electrodes. However, the heating to which the electrodesare subjected to remove gases therefrom does not ordinarily remove thesesubstances, since barium oxide and sodium chloride, in which form thesesubstances are usually present, are not vaporized from electrodes byheat alone unless the heat exceeds the melting point of nickel, which iscommonly used as the electrode material. Hence, in the ordinary processof evacuation, in which the temperature must be kept below that of themelting point of nickel, more or less barium oxide, sodium chloride orthe like, will remain.

It is the object of our invention to remove vaporizable electro-positivesubstances which would remain on normally non-emissive electrodes afterthe usual evacuation process.

The invention will be explained with reference to the drawing, in whichis shown diagrammatically a control rectifier and an apparatus employedin practicing our invention. In this drawing, 1 represents a controltube having the usual cathode 2. 3 is a grid or control element, and 4is the anode.

In evacuating tube 1 and its electrodes, the cathode 2 may be heatedfrom a current source 5 with or without a potential on the anode from asource 6 and the electrodes may also be heated inductively in the usualmanner, the gases being removed meanwhile by a pump connected to a stein'7. I 7' After the tube 1 has been evacuated, the oathode 2 isdisconnected, as by opening switch 8, an inert gas such as argon isintroduced through the stem 7, and a suitable source of periodicallyreversed current 9 is connected between the grid 3 and the anode 4, bysuitable means such as a switch 10. A series resistance 11 is introducedinto the circuit for the purpose of limiting the current fiow. Underordinary conditions we prefer a gas pressure of about 5 millimeters anda voltage in the grid-anode circuit sufiioient to cause an arc dischargebetween points on the grid 3 and the anode 4 where there are particlesof emissive material. Ordinarily the current is limited to from 2 to 3amperes. The current, applied voltage, and gas pressure depend to someextent on the electrode construction and the current rating of the tube.

The are so established vaporizes the emissive material at the spot whereit first forms and then moves from one particle to another over thesurfaces of the electrodes until all deposits have been vaporized. Thevapors finally condense on the tube walls or are absorbed by gettersubstances which may be present in the tube.

The gas employed should be such that the ionization potential thereof isgreater than the ionization potential of the objectionableelectropositive metals in order that the metals may ionize in preferenceto the gas and support the arc.

The applied potential and the gas pressure therefor should be so chosenthat they will not support an arc after all traces of electro-positivemetals have been removed from the grid and plate. In practice thiscondition is indicated by the are automatically changing to a glowdischarge.

Tubes treated in this manner have been found to be free of objectionablevaporizable substances and will preserve their characteristics, andtubes made in quantities will have uniform characteristics.

We claim:

1. The method of removing deposits of highly electro-positive metal fromnon-emissive electrodes in a discharge tube, which comprises applying aperiodically-reversed potential between two such electrodes with thecathode disconnected, admitting an inert gas to the tube at a pressurewhich will support an arc discharge between a particle of saidelectro-positive metal on one electrode and the other electrode, andcausing the arc to change to a glow discharge after saidv deposits havebeen vaporized.

2. In an electron discharge tube containing an emissive electrode and aplurality of non-emissive electrodes, the method of cleaning thenonemissive electrodes of deposits of emissive material, which comprisesfilling the tube with inert gas and vaporizing said deposits byestablishing an arc discharge therebetween, the emissive electrode beingunheated and disconnected.

3. In an electron discharge tube containing an emissive electrode and aplurality of non-emissive electrodes, the method of cleaning thenon-emissive electrodes of deposits of emissive material, whichcomprises filling the tube with a rare gas having a greater ionizationpotential 'thansaid deposits and vaporizing said deposits byestablishing an arc discharge therebetween.

4. The method of cleaning normally nonemissive electrodes of surfacedeposits of emissive material, which comprises causing an are betweensaid electrodes to travel over their surfaces from one particle toanother of the emissive material until all of said material has beenvaporized.

5. The method of cleaning normally nonemissive electrodes of surfacedeposits of objectionable electro-positive substances, which comprisescausing an are between said electrodes to travel over their surfacesfrom one particle to another of said substances and causing the arc tochange automatically to a glow discharge when all of said substanceshave been vaporized.

6. The method of removing deposits of objectionable electropositivesubstances from nonemissive electrodes in a discharge tube, whichcomprises applying a periodically-reversed potential between two suchelectrodes with the cathode disconnected and admitting a rare gas to thetube, the combination of the gas pressure and potential being chosen tostrike an arc between points on said electrodes where there are depositsof said substances, said gas having a higher ionization potential thanthe substances to be removed.

DONALD V. EDWARDS. EARL K. SMITH.

